1. Field of the Invention
The present invention relates generally to illumination systems of microlithographic projection exposure apparatuses. Such apparatuses are used to produce large-scale integrated circuits and other micro-structured components. More particularly, the invention relates to beam transforming devices in such illumination systems that modify the radial energy distribution of the illumination light bundle.
2. Description of Related Art
In the production of micro-structured components, a plurality of structured layers is applied to a suitable substrate, for example a silicon wafer. To structure the layers, these are first covered with a photosensitive resist. The resist is sensitive to light of a particular wavelength range, e.g. light in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) spectral range. The coated wafer is then exposed in a projection exposure apparatus that comprises an illumination system and a projection objective. The illumination system illuminates a mask that contains a pattern of structures to be formed on the wafer. The projection objective images the illuminated structures onto the resist. Since the magnification is generally less than 1, such projection objectives are often referred to as reduction objectives.
After the resist has been developed, the wafer is subjected to an etching or separating process. As a result of this process, the top layer is structured according to the pattern on the mask. The remaining resist is then removed from the other parts of the layer. This process is repeated until all layers are applied to the wafer.
The performance of the projection exposure apparatuses is not only determined by the imaging properties of the projection objective but also by the optical properties of the illumination system that illuminates the mask. The illumination system contains a light source, e.g. a pulsed laser, and a plurality of optical elements which generate a projection light bundle having the desired properties. These properties include the angular distribution of the light rays which form the projection light bundle. If the angular distribution of projection light is specifically adapted to the pattern contained in the mask, this pattern can be imaged with improved image quality onto the wafer covered with the photosensitive resist.
The angular distribution of projection light in the mask plane is often not described as such, but as an intensity distribution in a pupil plane. This exploits the fact that angles formed between the optical axis and light rays passing a field plane correspond to radial distances at which the respective light rays pass a pupil plane. In a so-called conventional illumination setting, for example, the region illuminated in such a pupil plane is a circular disc which is concentric with the optical axis. At each point in the field plane, light rays therefore impinge with angles of incidence between 0° and a maximum angle determined by the radius of the circular disc.
In so-called non-conventional illumination settings, e.g. ring field, dipole or quadrupole illuminations, the region illuminated in the pupil plane has the shape of a ring which is concentric with the optical axis, or a plurality of separate areas arranged off the optical axis. In these non-conventional illumination settings, only oblique rays illuminate the mask.
To generate an angular distribution of projection light that is optimally adapted to the mask, DUV illumination systems often employ an optical raster element, which can be for example a diffractive optical element (DOE) or a microlens array. Further examples of such raster elements are described in U.S. Pat. No. 6,285,443 assigned to the applicant. When changing between different illumination settings, e.g. from a conventional setting to a quadrupole setting, it is generally necessary to change the optical raster element. For fine tuning the angular distribution of illumination, and also to generate annular illumination settings, illumination systems often have a beam transforming device.
A typical beam transforming device for an illumination system of a microlithographic exposure apparatus is disclosed in EP 747 772 A. The beam transforming device is formed by a zoom axicon objective that combines a zoom function for the continuously variable adjustment of the diameter of a light distribution and an axicon function for the radial redistribution of light intensities. The axicon system comprises two mutually axially displaceable axicon elements having mutually facing conical refractive surfaces which can be moved towards one another until they are at zero spacing. By adjusting the zoom axicon objective, it is possible to set different annular intensity distributions in an exit pupil of the zoom axicon objective and, in conventional illumination settings, different degrees of coherence. A second optical raster element, which is located in the exit pupil of the zoom axicon objective, is illuminated with the light distribution generated by the first optical raster element and the zoom axicon objective, and produces an illuminated field in the mask plane.
Illumination systems with similar beam transforming devices for the radial redistribution of light energy are described, for example, in U.S. Pat. Nos. 5,675,401, 6,377,336 and 6,452,662, all assigned to the applicant. Further beam transforming devices comprising axially displaceable refractive optical elements are known from a variety of publications like U.S. Pat. Nos. 4,674,845, 4,997,250, 6,100,961, 6,377,336, 4,317,613 and GB 907 679 A, all of them comprising axially displaceable prisms or conic surfaces to change the cross-sections of the illumination light bundle.
With the recent tendency to higher throughput and shorter wavelengths, the current beam transforming devices of illumination systems suffer from several drawbacks. With the increasing demand in the performance of projection exposure apparatuses, also the demands for the illumination systems increases in almost every aspect, such as field and pupil homogeneity, pupil shape control, polarization control and reliability for high power systems With shorter wavelengths such as 193 nm, 157 nm and below, optical materials which can be used for the manufacture of lenses and other refractive optical elements are restricted. Some of them, e.g. CaF2, suffer from intrinsic birefringence. Other materials, such as SiO2, suffer from light induced birefringence effects. Furthermore, all materials need to be of high homogeneity and are thus expensive. Therefore it is advantageous to reduce the amount of transparent material in an illumination system.
A drawback of current illumination systems is further given by the restricted optical performance at higher numerical apertures. As the numerical aperture of projection optical systems ever and ever increases—numerical apertures of about NA>1.4 at wafer level are already under discussion—, also the maximum propagation angle of light rays with respect to the optical axis in the illumination system increases. This particularly makes the design of the beam transforming devices in such illumination systems more difficult.
Beam transforming devices of illumination systems transform the spatial distribution of the illumination light. In the ideal case, beam transforming devices thereby do not influence the angular distribution. With the spatial distribution of illumination light here the shape of the effective light source is described, which is imaged into the entrance pupil of the projection optical system. As has already been mentioned, size and shape of the effective light source, i.e. the angular distribution of the light in the mask plane, has a major impact on the imaging properties and the process control of the projection exposure apparatus. So far, beam transforming devices comprising refractive or reflective optical components suffer from the limited field invariance of the angular spectrum of the beam transformation, or, in other words, from the limited field invariance of the effective light source. Every field point in a field plane has to be illuminated, for example after scan integration, by identical imaging conditions and therefore also by identical illumination conditions determined by identical effective light source images for every field point.
The known double axicon or double prism beam transforming devices described above, however, create different effective light source distributions for different field points. This so-called axicon effect arises from the finite angular spectrum of light propagating through the axicon pair. For a perfect collimated bundle that propagates parallel to the optical axis, an axicon or prism pair perfectly transforms an incident parallel bundle in an exit parallel bundle of different spatial cross section. For a tilted bundle of rays, however, the effect of prism or axicon pairs on the spatial distribution becomes a complicated function of the distance of the prism or axicon pairs, and thus the spatial cross section of the bundle depends on the angle of the bundle formed to the optical axis and on the transformation of the cross-section of the illumination light.
In illumination systems for EUV microlithographic exposure apparatuses, refractive optical elements such as lenses, prisms or axicon elements, cannot be used because the available materials are not transparent for wavelengths in the range between 11 nm and 14 nm that are discussed in connection with EUV lithography.
U.S. Pat. No. 6,452,661 B1 discloses a purely reflective illumination system for an EUV projection exposure apparatus. The illumination system comprises a collimating mirror that collimates a diverging light bundle produced by a radiation source and directs this collimated light bundle onto an adjustable annular light beam converting unit. This unit converts the parallel light beam having a circular cross-section to a light beam having an annular (i.e. ring-shaped) cross-section. To this end, the light beam converting unit comprises a first reflecting member with a ring-shaped reflecting surface and a second reflecting member having a conical reflecting surface. To vary the ratio to the inner diameter of the ring to the outer diameter of the light beam, the first reflecting member and the second reflecting member are moved relative to one another.
There are various other purely reflective beam transforming devices known in the art. However, these devices are not adapted to the requirements of illumination systems for microlithographic exposure apparatuses, and in particular not suitable for being used in an EUV illumination system. For example, in a purely reflective EUV illumination system the angles of incidence have to be kept within certain ranges because only then reflectivities of more than 65% can be obtained with the current technology.
For example, U.S. Pat. No. 1,988,946 describes a diaphragm system for a microscope condenser. The condenser comprises two coaxial pairs of reflectors, wherein each pair consists of a male-cone and an female-cone reflector. The two pairs lie opposite each other and include means for axially displacing the two male-cones. This diaphragm system is arranged in a path of collimated light and makes it possible to vary the diameter of the parallel light bundle and to transform it into an emerging bundle having an annular shape.
U.S. Pat. No. 804,996 discloses a telescope comprising two reflective and rotationally symmetric mirrors. Parallel light is directed by the first mirror in such a way onto the second mirror that it emerges from the second mirror again as parallel light, but with a reduced diameter.
Purely reflective optical collectors without a beam transformation effect are disclosed, for example, in U.S. Pat. No. 3,817,605. This document relates to a device for converging sunlight beams so to be able to ignite a fire or do solar cooking. The device consists of two nested rotationally symmetric curved mirrors that converge parallel sunlight to a focal point.
U.S. Pat. No. 2,198,014 describes a headlight for motor vehicles in which light produced by lamp is collected and collimated by two nested rotationally symmetric mirrors having curved mirror surfaces.
WO 2005/031748 A1 and US 2003/0043455 A1 describe collectors for EUV illumination systems that efficiently transform large aperture angles produced by a radiation source into smaller aperture angles that can be reduced to zero by a subsequent collimator mirror.